Beyond Linux® From Scratch (systemd Edition) - Version r12.2-802

Chapter 4. Security

MIT Kerberos V5-1.21.3

Introduction to MIT Kerberos V5

MIT Kerberos V5 is a free implementation of Kerberos 5. Kerberos is a network authentication protocol. It centralizes the authentication database and uses kerberized applications to work with servers or services that support Kerberos allowing single logins and encrypted communication over internal networks or the Internet.

[Note]

Note

Development versions of BLFS may not build or run some packages properly if LFS or dependencies have been updated since the most recent stable versions of the books.

Package Information

  • Download (HTTP): https://kerberos.org/dist/krb5/1.21/krb5-1.21.3.tar.gz

  • Download MD5 sum: beb34d1dfc72ba0571ce72bed03e06eb

  • Download size: 8.7 MB

  • Estimated disk space required: 95 MB (add 14 MB for tests)

  • Estimated build time: 0.3 SBU (Using parallelism=4; add 1.1 SBU for tests)

MIT Kerberos V5 Dependencies

Optional

BIND Utilities-9.20.3, CrackLib-2.10.2 (/usr/share/dict/words referred by some tests), GnuPG-2.4.7 (to authenticate the package), keyutils-1.6.3, OpenLDAP-2.6.9, Valgrind-3.24.0 (used during the test suite), yasm-1.3.0, libedit, cmocka, kdcproxy, pyrad, and resolv_wrapper

[Note]

Note

Some sort of time synchronization facility on your system (like ntp-4.2.8p18) is required since Kerberos won't authenticate if there is a time difference between a kerberized client and the KDC server.

Installation of MIT Kerberos V5

Build MIT Kerberos V5 by running the following commands: 

cd src &&
sed -i -e '/eq 0/{N;s/12 //}' plugins/kdb/db2/libdb2/test/run.test &&

./configure --prefix=/usr            \
            --sysconfdir=/etc        \
            --localstatedir=/var/lib \
            --runstatedir=/run       \
            --with-system-et         \
            --with-system-ss         \
            --with-system-verto=no   \
            --enable-dns-for-realm   \
            --disable-rpath          &&
make

To test the build, issue: make -j1 -k check. Some tests may fail with the latest version of dejagnu and glibc. Some tests may hang for a long time and fail if the system is not connected to a network. One test, t_kadm5srv, is known to fail. If keyutils-1.6.3 is installed but Keyutils Kernel Configuration is not satisfied, some tests will fail complaining keyctl failed with code 1.

Now, as the root user: 

make install &&
cp -vfr ../doc -T /usr/share/doc/krb5-1.21.3

Command Explanations

The sed command removes a test that is known to fail.

--localstatedir=/var/lib: This option is used so that the Kerberos variable runtime data is located in /var/lib instead of /usr/var.

--runstatedir=/run: This option is used so that the Kerberos runtime state information is located in /run instead of the deprecated /var/run.

--with-system-et: This switch causes the build to use the system-installed versions of the error-table support software.

--with-system-ss: This switch causes the build to use the system-installed versions of the subsystem command-line interface software.

--with-system-verto=no: This switch fixes a bug in the package: it does not recognize its own verto library installed previously. This is not a problem, if reinstalling the same version, but if you are updating, the old library is used as system's one, instead of installing the new version.

--enable-dns-for-realm: This switch allows realms to be resolved using the DNS server.

--disable-rpath: This switch prevents hard coding library search paths (rpath) into the binary executable files and shared libraries. This package does not need rpath for an installation into the standard location, and rpath may sometimes cause unwanted effects or even security issues.

--with-ldap: Use this switch if you want to compile the OpenLDAP database backend module.

Configuring MIT Kerberos V5

Config Files

/etc/krb5.conf and /var/lib/krb5kdc/kdc.conf

Configuration Information

Kerberos Configuration
[Tip]

Tip

You should consider installing some sort of password checking dictionary so that you can configure the installation to only accept strong passwords. A suitable dictionary to use is shown in the CrackLib-2.10.2 instructions. Note that only one file can be used, but you can concatenate many files into one. The configuration file shown below assumes you have installed a dictionary to /usr/share/dict/words.

Create the Kerberos configuration file with the following commands issued by the root user: 

cat > /etc/krb5.conf << "EOF"
# Begin /etc/krb5.conf

[libdefaults]
    default_realm = <EXAMPLE.ORG>
    encrypt = true

[realms]
    <EXAMPLE.ORG> = {
        kdc = <belgarath.example.org>
        admin_server = <belgarath.example.org>
        dict_file = /usr/share/dict/words
    }

[domain_realm]
    .<example.org> = <EXAMPLE.ORG>

[logging]
    kdc = SYSLOG:INFO:AUTH
    admin_server = SYSLOG:INFO:AUTH
    default = SYSLOG:DEBUG:DAEMON

# End /etc/krb5.conf
EOF

You will need to substitute your domain and proper hostname for the occurrences of the <belgarath> and <example.org> names.

default_realm should be the name of your domain changed to ALL CAPS. This isn't required, but both Heimdal and MIT recommend it.

encrypt = true provides encryption of all traffic between kerberized clients and servers. It's not necessary and can be left off. If you leave it off, you can encrypt all traffic from the client to the server using a switch on the client program instead.

The [realms] parameters tell the client programs where to look for the KDC authentication services.

The [domain_realm] section maps a domain to a realm.

Create the KDC database: 

kdb5_util create -r <EXAMPLE.ORG> -s

Now you should populate the database with principals (users). For now, just use your regular login name or root. 

kadmin.local
kadmin.local: add_policy dict-only
kadmin.local: addprinc -policy dict-only <loginname>

The KDC server and any machine running kerberized server daemons must have a host key installed: 

kadmin.local: addprinc -randkey host/<belgarath.example.org>

After choosing the defaults when prompted, you will have to export the data to a keytab file: 

kadmin.local: ktadd host/<belgarath.example.org>

This should have created a file in /etc named krb5.keytab (Kerberos 5). This file should have 600 (root rw only) permissions. Keeping the keytab files from public access is crucial to the overall security of the Kerberos installation.

Exit the kadmin program (use quit or exit) and return back to the shell prompt. Start the KDC daemon manually, just to test out the installation: 

/usr/sbin/krb5kdc

Attempt to get a ticket with the following command: 

kinit <loginname>

You will be prompted for the password you created. After you get your ticket, you can list it with the following command: 

klist

Information about the ticket should be displayed on the screen.

To test the functionality of the keytab file, issue the following command as the root user: 

ktutil
ktutil: rkt /etc/krb5.keytab
ktutil: l

This should dump a list of the host principal, along with the encryption methods used to access the principal.

Create an empty ACL file that can be modified later: 

touch /var/lib/krb5kdc/kadm5.acl

At this point, if everything has been successful so far, you can feel fairly confident in the installation and configuration of the package.

Additional Information

For additional information consult the documentation for krb5-1.21.3 on which the above instructions are based.

Systemd Unit

If you want to start Kerberos services at boot, install the krb5.service unit included in the blfs-systemd-units-20240916 package using the following command: 

make install-krb5

Contents

Installed Programs: gss-client, gss-server, k5srvutil, kadmin, kadmin.local, kadmind, kdb5_ldap_util (optional), kdb5_util, kdestroy, kinit, klist, kpasswd, kprop, kpropd, kproplog, krb5-config, krb5-send-pr, krb5kdc, ksu, kswitch, ktutil, kvno, sclient, sim_client, sim_server, sserver, uuclient, and uuserver
Installed Libraries: libgssapi_krb5.so, libgssrpc.so, libk5crypto.so, libkadm5clnt_mit.so, libkadm5clnt.so, libkadm5srv_mit.so, libkadm5srv.so, libkdb_ldap.so (optional), libkdb5.so, libkrad.so, libkrb5.so, libkrb5support.so, libverto.so, and some plugins under the /usr/lib/krb5 tree
Installed Directories: /usr/include/{gssapi,gssrpc,kadm5,krb5}, /usr/lib/krb5, /usr/share/{doc/krb5-1.21.3,examples/krb5}, /var/lib/krb5kdc, and /run/krb5kdc

Short Descriptions

gss-client

is a GSSAPI test client

gss-server

is a GSSAPI test server

k5srvutil

is a host keytable manipulation utility

kadmin

is an utility used to make modifications to the Kerberos database

kadmin.local

is an utility similar to kadmin, but if the database is db2, the local client kadmin.local, is intended to run directly on the master KDC without Kerberos authentication

kadmind

is a server for administrative access to a Kerberos database

kdb5_ldap_util (optional)

allows an administrator to manage realms, Kerberos services and ticket policies

kdb5_util

is the KDC database utility

kdestroy

removes the current set of tickets

kinit

is used to authenticate to the Kerberos server as a principal and acquire a ticket granting ticket that can later be used to obtain tickets for other services

klist

reads and displays the current tickets in the credential cache

kpasswd

is a program for changing Kerberos 5 passwords

kprop

takes a principal database in a specified format and converts it into a stream of database records

kpropd

receives a database sent by kprop and writes it as a local database

kproplog

displays the contents of the KDC database update log to standard output

krb5-config

gives information on how to link programs against libraries

krb5kdc

is the Kerberos 5 server

krb5-send-pr

sends a problem report (PR) to a central support site

ksu

is the super user program using Kerberos protocol. Requires a properly configured /etc/shells and ~/.k5login containing principals authorized to become super users

kswitch

makes the specified credential cache the primary cache for the collection, if a cache collection is available

ktutil

is a program for managing Kerberos keytabs

kvno

prints keyversion numbers of Kerberos principals

sclient

is used to contact a sample server and authenticate to it using Kerberos 5 tickets, then display the server's response

sim_client

is a simple UDP-based sample client program, for demonstration

sim_server

is a simple UDP-based server application, for demonstration

sserver

is the sample Kerberos 5 server

uuclient

is another sample client

uuserver

is another sample server

libgssapi_krb5.so

contains the Generic Security Service Application Programming Interface (GSSAPI) functions which provides security services to callers in a generic fashion, supportable with a range of underlying mechanisms and technologies and hence allowing source-level portability of applications to different environments

libkadm5clnt.so

contains the administrative authentication and password checking functions required by Kerberos 5 client-side programs

libkadm5srv.so

contains the administrative authentication and password checking functions required by Kerberos 5 servers

libkdb5.so

is a Kerberos 5 authentication/authorization database access library

libkrad.so

contains the internal support library for RADIUS functionality

libkrb5.so

is an all-purpose Kerberos 5 library